In numerous medical and industrial applications, continuous in-line monitoring of a fluid is often necessary to ensure consistency of a process or to ensure safety. For example, the pressure of fluid in a line may be critical to a process. Additionally, the presence of air or other gas within a fluid or the presence of contaminants within a fluid may need to be monitored.
In the medical arena, gas-in-line detection systems are used to prevent the inadvertent infusion of gas into a patient's bloodstream. While small bubbles of gas may have no adverse effect on a patient, large gas bubbles can cause air embolism resulting in pain or death. Methods for the in-line detection of gas typically involve ultrasound or light transmission through the fluid line being monitored. The different transmission characteristics of sound or light through fluids and gases may be utilized to form an indication of the presence of a gas bubble in liquid in the fluid line. Simple recognizable perturbations of the signals from such sensors may be utilized to trigger an alarm and/or halt the infusion. Such systems require that the fluid and the associated conduit be substantially transparent to the energy being transmitted.
In one exemplary implementation, ultrasonic energy in the megahertz (MHz) range is coupled on one side of a conduit under test, and a receiver is placed on the opposite side. When a gas bubble is present in the conduit, energy is attenuated from the transmitted side to the received side. When fluid is present in the conduit, the energy received in the receiver is greatly increased. This energy or signal strength may thus be used as an indicator to determine whether gas is present in the conduit. Additionally, if the fluid rate is known, gas bubble size can be determined and thresholds can be set to indicate when a gas bubble has exceeded a preset limit, thereby triggering an alarm.
However, too often gas bubbles do not travel at the same velocity as the fluid, causing the gas bubbles to be interpreted as larger than they are, generating a false or nuisance alarm. This can be caused by a “Taylor” type bubble or “champagne” bubbles sticking to the side of the conduit, causing sufficient attenuation to cause an alarm. Additionally, ultrasonic or optical gas-in-line detectors typically cannot determine the exact size of gas bubbles and are configured merely to indicate the presence of gas bubbles which are greater than a predetermined size.
Other apparatus capable of detecting impurities such as gas within a fluid include optical systems. However, image processing used in conjunction with such optical systems make this option prohibitively expensive.